Thermally self-protected power switching semiconductor devices are known. For example, Hyink et al, U.S. Pat. Re. No. 30,514 shows a temperature sensitive thyristor thermally coupled to a power switching thyristor and shunting gate drive above a predetermined temperature to protect the latter by preventing turn-on. The temperature device is electrically connected in parallel with the gate of the power device and senses the temperature of the latter. In one embodiment, the thermal device may be of a semiconductor material such as germanium having a lower energy band gap than the semiconductor material of the power device such as silicon. The lower energy band gap of the thermal element insures that gate current will be shunted therethrough because of its lower junction voltage drop.
Whitney et al, U.S. Pat. No. 3,708,720 also discloses a silicon power device having a germanium thermal element in parallel with the gate for shunting purposes at elevated temperatures. The thermal element is provided by a pair of antiseries back to back germanium diodes. A DC gating source is used.
The present invention provides an improved thermally protected semiconductor switch, particularly affording accurate phase control even with AC gate drive. The accurate phase control is afforded by antiseries back to back diode junctions having nonsymmetrical leakage currents. At a given elevated temperature, leakage current flow through the diode pair is substantially greater in one direction than in the opposite direction. This enables forward gate drive to be shunted through the diode pair, but reverse gate drive to be blocked.
The latter blocking action prevents the draining of a reverse charged timing capacitor such that the latter is always at the same reference point when the positive half cycle starts. Since the phasing capacitor begins charging from the same reference level during each positive half cycle, the charging time will be the same, thus affording accurate phase control in firing the power switch.
In prior devices, without the noted nonsymmetrical leakage characteristics, one must tolerate the trade-off between protective shunting and phase control accuracy in AC gate applications. On the one hand, thermally induced leakage current must be great enough to insure shunting of forward gate drive, and thus afford protection for the power switch. On the other hand, minimal leakage current and large blocking capability is desired in the other half cycle so as to prevent drainage of a reverse gate drive charged phasing capacitor. The present invention affords both of these heretofore incompatible results.